feat: don't consider "extensions" for directories when sorting

```bash
$ mkdir a b.m c.n d.m e ; touch f.txt g.png h.png

$ eza --group-directories-first                              
a  b.m  c.n  d.m  e  f.txt  g.png  h.png

$ eza --group-directories-first --sort=extension
a  e  b.m  d.m  c.n  g.png  h.png  f.txt
```

The directories are also being sorted by extension based on the dot in the names. even though directories don't have extensions.